The present invention generally relates to image forming apparatuses, and more particularly to an image forming apparatus such as a laser beam printer and a digital copying machine.
In an image forming apparatus such as a laser printer, the ON/OFF state of a laser diode is controlled depending on a video signal which is synchronized to a pixel clock signal, so as to modulate a laser beam which is emitted from the laser diode by the video signal. The laser beam is periodically deflected by a polygonal mirror, for example, and scans a photoconductive or photosensitive body so as to form an image. Generally, the photoconductive or photosensitive body is shaped in a form of a drum or a belt.
According to a well known optical scanning method using the laser beam, the laser beam emitted from the laser diode is deflected by the polygonal mirror to scan the photoconductive drum in a main scan direction. At the same time, the photoconductive drum is rotated a predetermined angle in a direction perpendicular to the main scan direction, that is, in a sub scan direction.
There is a conventional image forming apparatus which is provided with a function of generating a test video signal which is related to a test pattern. By carrying out the image forming apparatus responsive to this test video signal, it is possible to check the operating state of the image forming apparatus and the picture quality which can be obtained.
However, the conventional image forming apparatus which has the function of generating the test video signal uses a test pattern generator circuit for generating the test video signal. For this reason, the test pattern which is described by the test video signal generated from the test pattern generator circuit is a fixed repetition pattern having four dots in the main scan direction and four dots in the sub scan direction, for example.
For this reason, in order to modify the kind of test pattern to be used, it is necessary to change the test pattern generator circuit. As a result, there is a problem in that the test pattern cannot be modified with ease and the flexibility of the image forming apparatus is poor.